This application relates to a regenerative thermal oxidizer having two heat exchangers which is operated continuously and efficiently to clean a process gas.
Regenerative thermal oxidizers are known in the prior art, and are often used to remove impurities or pollutants from an industrial gas stream. In one example, an air stream containing volatile organic compounds such as found in air from a paint spray booth, is directed through a regenerative thermal oxidizer ("RTO"). The RTO removes the impurities from the air stream. In a standard regenerative thermal oxidizer, there are at least two heat exchangers, with a first heat exchanger typically receiving a cool gas to be cleaned, or a "dirty" gas. The other heat exchanger is receiving a hot, clean gas from a combustion chamber.
The dirty gas to be cleaned passes through the first heat exchanger, which is in an inlet mode, and into the combustion chamber. The gas is combusted and cleaned in the combustion chamber. At the same time, gas continuously moves out of the combustion chamber through the second heat exchanger, which is in an outlet mode, and into an outlet passage leading to atmosphere.
The air leaving the combustion chamber is quite hot, and it heats the second heat exchanger. At the same time, the dirty gas passing through the first heat exchanger is relatively cool, and it cools the first heat exchanger. After a period of time, valves associated with the two heat exchangers are switched such that the first heat exchanger, which had previously received the cool, dirty gas, is switched to receiving the hot, clean gas. The second heat exchanger which had been receiving the hot, clean gas is switched to receiving the dirty, cool gas. The dirty, cool gas now passes over the previously heated heat exchanger and is preheated prior to reaching the combustion chamber. The preheating improves the efficiency of the system. At the same time, the heat exchanger which had been previously receiving the cool, dirty gas, and which has been cooled by that gas, is now again heated by the hot, clean gas. This cyclical process is repeated as the RTO continuously and efficiently removes impurities from a high volume industrial gas stream.
One problem encountered with RTO systems is that since the outlet passage is typically directed back to atmosphere, strict controls are necessary to insure that no "dirty" gas reaches the atmosphere. When a heat exchanger is initially switched from being in an inlet mode where it receives dirty gas, to being in an outlet mode where it receives clean gas, there may be some residual dirty gas remaining in the heat exchanger.
To address this problem, RTO's have often been provided with a third heat exchanger. The third heat exchanger is operated in a purge mode to drive any residual dirty gas from the heat exchanger, prior to that heat exchanger moving to an outlet mode. Purge modes are typically included on RTO systems having three or more heat exchangers.
In some cases it might be desirable to have an RTO system with only two heat exchangers. The purge function would still be desirable to minimize the flow of dirty gas to atmosphere. It has typically been believed that a break in flow of gas from the source of dirty gas is necessary during the time the purge drives residual air from the heat exchanger in a two heat exchanger RTO system. It is a goal of any RTO system to maximize the volume of gas that is cleaned. Providing a break between the inlet and outlet modes undesirably decreases the efficiency. Also, it is desirable to continue to process dirty gas continuously and move the dirty gas from the industrial source of dirty gas to the RTO continuously.
Thus, there has been some effort to develop an RTO system wherein two heat exchangers are provided with a purge function. One example is shown in the PCT International Patent Publication No. WO91/00477. In this disclosure, a single rotary valve alternately connects two heat exchangers between an inlet and an outlet passage. The inlet passage is alternately connected to the dirty gas, or to a clean purging gas. The single rotary valve must fully separate the inlet and outlet passages. The operation of this system is such that the single rotary valve would have to be controlled extremely accurately to prevent the communication of dirty gas with the outlet passage. Even with careful control, leakage of dirty gas to the outlet is a possibility. As such, this proposed system does not achieve all of the goals for a two heat exchanger RTO system.